Adding test for Number.prototype.toFixed
Fixing tests so they really run

Commented out a test because I don't know how to get more precise than toString.
Making a function that's closer to spec.

It still does not handle
	(1000000000000000128).toFixed(0) === "1000000000000000128"

I don't know how to get more precise than .toString().  I suppose that I'd rather be as close as JS allows instead of consistently failing.